1. Field of the Invention
The present invention is generally related to optical heterodyne wavefront sensors, and more particularly to an optical heterodyne wavefront sensor that uses heterodyne interferometry to measure a two dimensional optical wavefront.
2. Description of the Prior Art
A conventional laser beam wavefront sensor responds to a two dimensional light beam and uses a camera to record an interferogram of the phase of the beam across its aperture. The phase information is used to correct aberrated beams, such as may occur when a light beam is propagated and steered through a long distance or through a turbulent atmosphere. The camera is usually read serially, and thus requires a relatively long period of time to produce a result. This is undesirable for high speed applications or for rapidly changing wavefronts.
An example of a heterodyne interferometer system is found in U.S. Pat. No. 4,688,940, "Heterodyne Interferometer System", invented by Gary E. Sommargren and Moshe Schaham. As disclosed, the heterodyne interferometer system utilizes a single stabilized frequency linearly polarized laser input beam from a light source. This is provided to an acousto-optic device along with a frequency stabilized electrical reference signal for transforming the input beam into a pair of orthogonally polarized beams. The beams differ in frequency by the reference signal frequency prior to providing the beams to a polarization type interferometer. A mixing polarizer mixes the beams after they traverse the interferometer and provides the mixed beams to a photoelectric detector where they are processed into an electrical measurement signal. The electrical measurement signal is processed in a phase meter/accumulator along with the reference signal to produce an output signal which is the sum of the single resultant phase difference on a cycle-by-cycle basis between the measurement signal and the reference signal. The phase meter/accumulator includes an analog-to-digital converter and a memory register for the previous cycle, with the measurement resolution being determined by the number of bits of the analog-to-digital converter. The system, however, does not measure the phase of an entire field or serve to correct the individual phase differences of the array of subapertures that constitute the aberrated beam, such as one which has propagated through a turbulent environment where the phase of each subaperture across the beam varies.
What is needed, therefore, is an optical heterodyne wavefront sensor that is capable of simultaneously measuring the phase of each subaperture across an optical wavefront, that allows two dimensional measurements of the state of phase of the wavefront in parallel, and that is capable of measuring the optical phases of more than one wavelength.